I assume your child is FF since you bought a Frontier. If that is the case, I wouldn't waste money on a convertible like the Radian. Try a Nautilus? http://www.car-seat.org/showthread.php?t=47220 (Scroll down about halfway... looks like someone had great luck with a Nauti in their 07 Passat.)

Oops failed to mention that ds1, who is 4.5 will be using it. But ds2 will use it very soon as I have a Marathon and Decathlon that will expire next May. Ds2 is only 16 months so I will have to have at least one convertible seat forvds to rf in. Ds1 RF until he was 3 and 33 pounds. if I had the radian or another convertible that had a higher rf weight limit then he could stay rf much longer. Is the Nautilus convertible?